Electro-optic device comprising an optic image relay and method of manufacturing same

ABSTRACT

An electro-optical device including a writing beam, a reading beam and an optic relay comprising a plate of a material having double refraction. According to the invention, a direct voltage is applied between the faces of the plate, the information being written by modulation of the writing beam. Application in information processing, display of data and the projection of images.

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XR 3 792 25a v ---vvw uwuwwn 9 i m 9 \X/ 14 I 4 1 Feb. 12, 1974 Donjonet al.

[ ELECTRO-OPTIC DEVICE COMPRISING 3,609,002 9/1971 Fraser 350/150 ANOPTIC IMAGE RELAY AND METHOD 2 2332; 3 :1 323x28 ar e OF MANUFACTURINGSAME 3,449,583 6/1969 Edeni 350/150 [75] Inventors: Jacques Donjon,Yerres; Jean-Pierre 3,252,000 5/1966 Mc y 350/ 150 Hala St Mau GerardJoseph Buchan Marcel Marie, LHay Les Roses, all i of France PrimaryExaminer-James W. Lawrence [73] Assignee: U.S. Philips Corporation, NewAssistant Nelms York Attorney, Agent, or FirmFrank R. Trifari [22]Filed: Mar. 28, 1972 [21] Appl, No: 238,836

, [57] ABSTRACT [30] Foreign Application Priority Data v I Anelectro-optical device Including a writing beam, a Mar.3l, 197] France..-,7lll3l9 reading beam and an optic relay comprising a plate of [52] US Cl 250/213 VT 350/150 I a material having double refraction. 51 Int.Cl. n01 j 39/12 According to the invention, a direct voltage is pp [5Fid f S h 250 2 3 225; 350 50; between the faces of the plate, theinformation being 313 03 R 103 A written by modulation of the writingbeam.

Application in information processing, display of data [56] ReferencesCited and the projection of images.

UNITED STATES PATENTS 3,499,157 3/1970 Satake 250/213 VT 4 Claims, 4Drawing Figures ELECTRO-OPTIC DEVICE COMPRISING AN OPTIC IMAGE RELAY ANDMETHOD OF MANUFACTURING SAME The present invention relates to anelectro-optic device comprising: at least a first source of a firstlight radiation, means for projecting the said radiation, at least asecond source of a second light radiation, means for polarizing andprojecting the said second light radiation, and an optic image relayconstituted by an evacuated enclosure having at least one window whichis transparent to the said light radiations, by a layer which isphoto-sensitive to the said first radiation, by a plate havingelectro-optic effect brought at a temperature near its Curie point andpresenting a double refraction which is variable as a function of thepotential difference existing between its faces, a first electrode whichis electrically conductive and optically transparent to the said secondlight radiation being disposed against the said plate and a secondelectrode being placed on the opposite side in the proximity of the saidplate.

An optic relay of this type is described in the Canadian applicationSer. No. 118.174, British application Ser. No. 33059-71 Japaneseapplication Ser. No. 52854-71, US. application Ser. No. 162.565 filedJuly 14, 1971 for Optic relay for scanning by means of a light beamfiled in the name of applicant.

The invention also relates to the application of the device permittingthe addition and the subtraction of optic images.

ln the above-mentioned patent application, the photosensitive layer maybe constituted, according to a first embodiment, by a photocathode. Inthis case, the second electrode facilitating the application of apotential difference between the faces of the plate is constituted by agrid placed at a few tens of microns from the second face of the plate,which second face is then covered with a secondary emission layer, thesaturation emission coefficient of which is higher than i, with theinterposition of an insulating mirror which reflects the second lightradiation. When a light beam impinges upon the photocathode, it emitselectrons which bombard the above secondary emission layer, under theinfluence of an accelerating voltage of 200 to 2,000 Volts appliedbetween the photocathode and the above grid, the focusing being obtainedby the proximity effect.

In a second embodiment, the photosensitive layer is constituted by aphotoconductive layer which becomes conductive under the influence ofthe first light radiation. Said layer is deposited on the second face ofthe plate with the interposition of an insulating mirror which reflectsthe second light radiation. The second electrode is then constituted bya conducting layer which is transparent to the first light radiation andis deposited on the photoconductive layer.

ln these two alternatives, the only object of the light beam is toproduce the electric contact between the second electrode and thecorresponding point of the second face of the plate, thus playing a partof flying short circuit, the image signal or the information signalbeing applied between the two electrodes. The first and second lightbeams may be of the same nature, that is to say having substantiallyidentical spectra in the visible range. lt may also relate to lightbeams having different wave lengths, for example, in the ultraviolet andvisible spectra.

ln all these devices, the writing on the second face of the plate of acharge image automatically erases the image which was writtenpreviously, no matterwhat the differences are existing between the twowritings.

According to the present invention, the electro-optic device comprising:at least a first source of a first light radiation, means for projectingthe said radiation, at

least a second source ofa second light radiation, means for polarizingand projecting the said second light radiation and an optic image relayconstituted by an evacuated enclosure having at least one window whichis transparent to the said light radiations, by a layer which isphotosensitive to the said first radiation, by a plate having anelectro-optic effect brought at a temperature in the proximity of itsCurie point and presenting a double refraction which is variable as afunction of the potential difference existing between its faces, a firstelectrode which is electrically conductive and optically transparent tothe said second light radiation being disposed against the said plateand a second electrode being placed on the opposite side in theproximity of the said plate, is characterized in that modulation meansof the said first light radiation are disposed before its incidence onthe said photosensitive layer, the said electrodes being connected to adirect voltage source.

The invention also relates to a method of manufacturing a device asdescribed above for the algebraic addition-of optic images,characterized in that the images transported by the first lightradiation are written on the said photosensitive layer, each of theoptical images being transformed into a charge image observed by meansof a second light radiation by variation of the double refraction.

The invention will be better understood during the following descriptionof particular embodiments with reference to the figures, which show:

FIG. I, a first embodiment of the invention in which the photosensitivelayer is a photoconductive layer scanned by a modulated light beam,

FIG. 2, a second embodiment of the invention in which the photosensitivelayer is a photoconductive layer scanned by a modulated light beam,

FIG. 3 a third embodiment of the invention in which the photosensitivelayer is a photocathode on which an image is projected.

FlG. 4, a fourth embodiment of the invention in which the photosensitivelayer is a photoconductive layer on which an image is projected.

In FlG. l, the optic image relay is provided in an evacuated enclosure10 which has two windows 8 and 11, respectively. Said windows may betransparent to the radiations of different wave lengths. A photocathode7 is deposited on the window 8. Opposite to the cathode 7 is placed theplate 1 having variable double refraction which may be a monocrystal ofdeuterated diacid potassium phosphate. Said plate is covered by aninsulating mirror 4 and by a secondary emission layer 5 the saturationemission coefficient of which is higher than 1. A grid 6 is placedbetween the photocathode 7 and the layer 5 at a distance of a few tensof microns. The outer face of the plate I is covered by a transparentconductive layer 3 and is glued on a transparent support 2, for exampleof calcium fluoride or barium fluoride, which has on the one hand theproperty of being isotropic and on the other hand of having a goodthermal conductivity. The plate is brought at a temperature nearitsCurie point by means of a cooling member 9.

In the first embodiment shown, the image projected on the photocathodeis written sequentially by means ofa light beam emanating from a sourcewhich may be, for example, a laser source, said beam being modulated byan electro-optic cell 21 and set into scanning motion by a lightdeflector 22. Said assembly may moreover be replaced by a flying spottube associated with an objective, the modulation of the light beingthen realised by the application of the signal voltage between thecathode and the Wehnelt of the tube while superimposing a suitabledirect voltage.

According to the invention, a direct voltage which may be positive ornegative is applied between the transparent conductive layer 3 and thegrid 6. The operation of the optic relay is not symmetric according tothe polarity. When the grid 6 is negative relative to the target, thesecondary emission coefficient of the layer 5 is very low and thedeposited charges are negative and practically equal to the number ofelectrons emitted by the photocathode 7 under the effect of the lightbeam 23. When the grid 6 is positive relative to the target, thesecondary emission coefficient 1; of the layer 5 is higher than 1 andfor simplification it may be considered that the charges deposited onthe bom barded face are positive charges proportional to (1 I). So alayer 5 must be chosen which presents a secondary emission coefficientwhich varies very little in the interval of accelerating voltages of theelectrons at the level of the target, of which interval the width is ofthe order of a hundred volts. As long as the potential differencesbetween the two faces of the plate 1 are lower than the direct voltageapplied between the two electrodes 3 and 6, the electric chargesdeposited in each point are proportional to the product of the luminousflux by the duration of its passage in each point. In this manner anaddition function is obtained by accummulation of charges depositedduring each scan.

In FIG. 2, elements identical to those of FIG. 1 have the same referencenumerals. The photosensitive element is constituted by a photoconductivelayer 16 whichis deposited on the insulating mirror 4. Thephotoconducting layer is covered by a conductive layer 17 which istransparent to the radiations issued by the source which in this case isa flying spot tube 24 associated with an objective 25, which source maybe, as above, a laser source. The direct voltage is applied between thetwo transparent conductive layers 3 and 17. The arrival of the luminousflux on the photoconductor l6 creates electron-hole pairs which modifythe charge state of the plate on the side of the mirror, as in the caseof FIG. 1; as long as the potential differences between the two faces ofthe plate 1 are lower than the direct voltage applied between the twoelectrons 3 and 17, the electric charges deposited in each point areproportional to the product of the luminous flux by the dura- In FIG. 4,elements identical to those of FIG. 2 have the same reference numerals.The image projected on the photoconductive layer may originate from anydevice, as in the case of FIG. 3.

In the case of FIGS. 3 and 4, as long as the potential differencesbetween the two faces of the plate are lower than the direct voltageapplied between the two electrodes 3 and 6 or 3 and 17, the electriccharges deposited in each point are proportional to the product of theluminous flux arriving at said point by the exposure time. In thismanner an integrating function is obtained by accummulation of chargesdeposited during the exposure.

In the embodiments of FIGS. 1 to 4 it is sufficient, to erase thewritten picture, to expose the photosensitive layer 7 or 16 andsimultaneously remove the voltage applied between the two electrodes 3and 6 or 3 and 17. The functioning of the system is then equivalent tothe writing of a zero signal according to the functioning described inthe above patent application. It is possible to use for the erasing asecond light beam which illuminates simultaneously all the points of thephotosensitive layer or the same optic scanning as for the writing inthe case of FIGS. 1 and 2. When said scanning is of the random type, aselective erasing may be obtained while directing the non-modulated beam23 only on the'de sirable points. When the scanningis of the recurrenttype, the selective erasing may be obtained while adding an on-offmodulation of the beam.

It is possible to add on the target the charges corresponding to severalsuccessive images thereby increasing the ratio between the level of therecurrent components and the levels of the non-recurrent components ofthe various images. The average image of several successive images isthus realized. As a possible application may be mentioned: theaugmentation of the signal-to-noise ratio of images having a importantnoise level, by integration of several successive images; other example:the display of elements common to several images corresponding to thesmoothing of a scene in different conditions (different angles ofshooting).

The reading of the information recorded in the optical image relay isdone, as in the said patent application, by observation of the plate orby projection of the image of said plate while using, as shown in FIGS.1 to 4, a light beam emitted by a source 30 which traverses a polarizer31, is reflected on the dielectric mirror 4 and traverses the analyser32, the separator 33 controlling the incident and reflected beams. Whensaid separator 33 is polarized, it also plays the part of the elements31 and 32. It is known that the image thus obtained, the luminosity ofwhich depends only upon the power of the source 30,,may be projected ona screen of large dimensions.

It is also possible to realize with the said optical image relay thesubtraction of images. Said subtraction is done as follows: The platebeing not charged initially, two successive series of images are formedon it. Between the two series, the polarity of the voltage appliedbetween the two electrodes 3 and 6 or 3 and 17 is inverted. Thus adistribution of charges is obtained which is equal to the differencebetween the sums of the charges corresponding to the two series ofimages. In the case where a photocathode and a grid 7 are used, however,the ratio between the efficacies of the deposit of the positive andnegative charges should be taken into account, which ratio is equal to(1; l), where "q represents the secondary emission coefficient of thelayer 5 during the sequence where the potential of the grid 6 is higherthan that of the target. In order to take said factor into account, itis sufficient to suitably control the average intensity of the lightbeam for each writing sequence. Said mode of functioning permits, forexample, of effecting the subtraction of two images, that is to say toeliminate all their identical parts (for example, fixed echos in radar).

It is to be noted that it is possible to amplify the received luminousflux while introducing in the optic relay an amplifier comprising aphotocathode and a luminescent material as described in the abovementioned patent application. In the case of the first and thirdembodiments, it is also possible to amplify .the electron beam whileintroducing a micro-channel multiplier as described in theabove-mentioned patent application.

When the optic image relay is exposed to coherent light, the device forthe optical information processing may be used which is described in theearlier mentioned application in the name of applicant for Apparatus foroptical information processing; on the track of the reading light beamis placed a phase transformer which introduces a phase shift equivalentto the addition, in each point, of a constant signal of a sign oppositeto that of the image signal. As in the earlier mentioned application no.said phase transformer may be constituted by a device exterior of theoptic relay, such as a compensator or an electro-optical cell, or evenmay be formed integral with the crystalline plate of the optic relay; itis sufficient, for this purpose in the case of the subtraction of imagesto choose a uniform exposure for one of the two writing frequencies. Aparticularly remakable mode of functioning and in accordance with thatdescribed in the earier mentioned application is that for which theintroduced constant phase shift is such that the average transmission ofthe assembly, in amplitude, is zero, facilitates cancellation of thezero order of diffraction in the Fourier transform of the image. Asecond remarkable mode of functioning is that for which the introducedconstant phase shift is equal in absolute value, and of opposite sign,to the phase shift of the most illuminated written image point. Theimage observed upon reading is then a negative of the image projected onthe photosensitive layer.

As the electro-optic-effect used upon reading does not destroy thecoherence of the light, the optic image relay described in the presentinvention permits the transformation of any image with a direct orinverted filter which may be used for the optic information processingwith coherent or incoherent light. It also permits the transformationof'any image in a positive or negative image of coherent light or ofincoherent light having a spectrum different from that of the originalimage.

What is claimed is:

l. A method of algebraically adding images in an optic image relay ofthe type wherein an image pattern optically projected on aphotosensitive layer'by a first source of light radiation is convertedthereby intoan electrical pattern on an electrically biassedelectrooptic plate interrogated by a second source of radiation,comprising the steps of first projecting an image pattern on thephotosensitive layer with the radiation level of the first source oflight radiation and the electrical bias level on the electro-optic platehaving intensities sufficient to produce a partially induced doublerefraction in the electro-optic plate, and repeating the first step aplurality of times.

2. A method as claimed in claim 1, further comprising the step ofreversing the electrical bias level on the electro-optic plate and thenexposing the photosensitive layer to a plurality of second imagepatterns whereby the induced double refraction in the electrooptic platecorresponds to the difference between the first and second imagepattern.

3. A method as claimed in claim 2, wherein the first pattern on thephotosensitive layer is spatially unmodulated during one of the twowriting sequencies at a value corresponding to an induced doublerefraction equal, in absolute value, to the maximum double refractioninduced at the brightest point during the other sequence, whereby thenegative image corresponding to said other sequence is produced.

4. A method as claimed in claim 2, wherein the first pattern on thephotosensitive layer is spatially unmodulated during one of the twowriting sequencies at a value corresponding to an induced doublerefraction equal, in absolute value, to the induced average doublerefraction during the other sequence, whereby a zero averagetransmission, in amplitude, of the second light radiation is obtainedand the zero order of diffraction in the Fourier transform of the imagewhich may be formed with said second radiation is eliminated.

1. A method of algebraically adding images in an optic image relay ofthe type wherein an image pattern optically projected on aphotosensitive layer by a first source of light radiation is convertedthereby into an electrical pattern on an electrically biassedelectro-optic plate interrogated by a second source of radiation,comprising the steps of first projecting an image pattern on thephotosensitive layer with the radiation level of the first source oflight radiation and the electrical bias level on the electro-optic platehaving intensities sufficient to produce a partially induced doublerefraction in the electrooptic plate, and repeating the first step aplurality of times.
 2. A method as claimed in claim 1, furthercomprising the step of reversing the electrical bias level on theelectro-optic plate and then exposing the photosensitive layer to aplurality of second image patterns whereby the induced double refractionin the electro-optic plate corresponds to the difference between thefirst and second image pattern.
 3. A method as claimed in claim 2,wherein the first pattern on the photosensitive layer is spatiallyunmodulated during one of the two writing sequencies at a valuecorresponding to an induced double refraction equal, in absolute value,to the maximum double refraction induced at the brightest point duringthe other sequence, whereby the negative image corresponding to saidother sequence is produced.
 4. A method as claimed in claim 2, whereinthe first pattern on the photosensitive layer is spatially unmodulatedduring one of the two writing sequencies at a value corresponding to aninduced double refraction equal, in absolute value, to the inducedaverage double refraction during the other sequence, whereby a zeroaverage transmission, in amplitude, of the second light radiation isobtained and the zero order of diffraction in the Fourier transform ofthe image which may be Formed with said second radiation is eliminated.